Kosovo police on Saturday said they had stopped from entering documentation that Serbia had sent to allow the ethnic Serb minority to take part in a referendum.
A statement said that one car and two trucks were stopped at the Merdare border crossing point with Serbia a day earlier. The trucks have been confiscated while six people in the vehicles were turned back.
Serbia is holding a referendum on Sunday on amendments to boost the independence of its judiciary as part of reforms needed for the country to move closer to European Union membership.
Belgrade wants its ethnic Serb minority in Kosovo to participate.
But Kosovo authorities say ethnic Serbs in its territory may cast the ballot only via mail or at a liaison office, ignoring past practice of setting up voting stations in Serb-dominated areas.
A statement on Friday from Kosovo's top authorities said Kosovo laws "do not recognize the right of one state to hold a referendum in the sovereign territory of another state," adding that "the practices applied so far since 2012 have been unconstitutional."
The decision is likely further strain relations between Kosovo and Serbia. Kosovo declared independence from Serbia in 2008, a move that Belgrade has refused to recognize.
Serbia has insisted that Kosovo remains part of the country, despite its declaration of independence following a 1998-99 conflict that killed some 13,000 and ended after NATO bombed Serbia to stop its crackdown on separatist ethnic Albanians.
Serbia has maintained a strong influence in Serb-dominated areas of Kosovo where tens of thousands of Serbs live, although it formally has no authority in the territory.
The dispute between Serbia and Kosovo remains a cause of tensions in the Balkans. EU-mediated negotiations aimed at normalizing relations have produced little progress, although both Kosovo and Serbia have been told to resolve their differences in order to move forward in their bids to join the EU.